Volkswagens economical diesel-powered Jetta TDI and new GT models made their U.S. debut. In May, two late-arriving specialty models were also released--the Golf Trek and Jetta Trek--both equipped to appeal to young Gen-Xers and others caught up in the mountain-bike craze. The TDI features VWs 90-horsepower, 1.9-liter direct injection diesel, mated to a 5-speed manual transmission. The combo produces eye-popping fuel economy of 40 mpg in the city and 49 mpg on the highway. The new Jetta GT includ...